The present invention relates to a normally-closed switch in which a movable contact member is normally held in contact with a fixed contact member by spring-load of a plate spring member. When an operating member is pressingly operated, the plate spring member is resiliently displaced into a position where the plate spring member comes in contact with a stopper member. The movable contact member is thus separated from the fixed contact member. When the operating member is pressed further, the plate spring member is not displaced, but a resilient member is compressingly deformed, thereby compensating for the over-stroke of the operating member.
There are several normally-closed switching means of this type employed in push-button dialing telephones.
In a push-button dialing telephone, the operating member of normally-closed switching means is pressed downwardly by a push-button or key top being depressed. The operating members moves the movable contact member, through the plate spring member, in the switch opening direction, thus causing the microphone circuit of the receiver to be shut off. Thereafter, a normally-opened switch is further operated, and closed, by the push of key tops to enable the circuit for detecting which numbers on the dial pad correspond to pushed key tops. The operating member of the normally-closed switching means is pushed further in this process than the distance corresponding to the distance to which key tops are pushed down to close the contacts of the normally-opened switch.
In the case of normally-closed switching means in which the operating member is over-travelled or over-stroked as described above, it is not enough for the operating member to travel only that distance necessary to separate the movable contact member from a fixed contact member. Rather, it is necessary for the operating member to over-travel a predetermined distance even after contacts of the normally-closed switch are opened.
Among the conventional normally-closed switching means, in which the operating member is arranged to make an over-stroke, are those in which the operating member and plate spring member are rigidly connected with each other in such a way that the plate spring member is resiliently deformed to such an extent as to correspond to the distance of the over-stroke of the operating member.
However, with such conventional means the plate spring member was unnecessarily deformed more than the amount of deformation otherwise necessary to separate the movable contact member from the fixed contact member. This arrangement has a serious drawback in that the plate spring member is so severely worn as to have an extremely short life, although it is intended to be used for a long time. In addition, the drawback is more prominent because the normally-closed switching means must be small in size considering the use thereof and the plate spring member employed in the normally-closed switching means must also be small in size. If a material having excellent anti-fatigue properties is employed to form the plate spring member, the drawback could be eliminated. However, such material is extremely high in cost and therefore neither proper nor practical for use as the material from which the plate spring member is made.